Flexible circuit card with laser-contoured VIAs and machined capacitors

ABSTRACT

A flexible circuit such as a membrane probe (10) is made by forming a trench (50, 60) in the upper surface (39) of a polyimide substrate (38) with a trench base (52, 62) spaced below the upper surface. The trench has an end wall (54, 64) ramped at an obtuse angle to the substrate upper surface and the trench base. A conductive layer deposited on the upper surface is patterned to form a line trace (44, 46) extending continuously over the substrate upper surface, down the ramped end wall and along the trench base, to contact a ground plane or form a distributed capacitance. An excimer laser is used, at a wavelength of 308 nm., an energy density less than 0.54 J./cm 2  (preferably 0.18 to 0.35 J./cm 2 ), and a pulse frequency of about 100 Hz., to ablate successive incremental thicknesses (80) of polyimide from the substrate in sweeps of depthwise decreasing length.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of copending application Ser. No. 07/463,695 filedon Jan. 11, 1990, now U.S. Pat. No. 5,066,357, issued Nov. 19, 1991.

BACKGROUND OF THE INVENTION

This invention relates generally to flexible circuits such as membraneprobe cards and more particularly to the formation of electrical circuitconductors and connections thereon.

Testing integrated circuits at the wafer level is essential for theeconomic manufacture of these complex devices. By rejecting defectivecomponents at an early stage, unnecessary packaging costs are avoided.Also, wafer test data provides early feedback on the overall status ofthe IC fabrication process so that deviations can be quickly detectedand corrected. VLSI technology places new demands on wafer test hardwareas the technology increases in level of integration and operatingspeeds. A critical component limiting the performance of the test systemis the wafer probe card.

A conventional probe card comprises a printed circuit board supportingan array of delicate, wire contact-styli which provide anelectromechanical interface between a device under test (DUT) and thetest electronics system. A major limitation of these probes is that theelectrical environment of the interface is poorly controlled. Each wirestylus acts essentially as a lumped parasitic conductance of up to 20nH. The result is to severely degrade signal fidelity at the DUT forfrequencies above 50 MHz due to cross talk and high frequencyattenuation. Also, these contact wires are so fragile that they do notmaintain positional stability during service, so frequent maintenanceand realignment are required for reliable operation. The alignmentprocedure becomes increasingly difficult with increasing pin count.

A membrane probe has been developed in an attempt to solve some of theseproblems. One of the earliest descriptions of a flexible probe waspublished in IBM Technical Disclosure Bulletin, Vol. 10, No. 10, March1968, pages 166-67, entitled, "Film Supported Probe for the AC PulseTesting of Integrated Circuits." This disclosure was elaborated insubsequent IBM Technical Disclosure Bulletins Vol. 13, No. 5, October1970, pages 1263-64, entitled, "Membrane Type Probe" and Vol. 15, No. 5,October 1972, pages 1513, entitled "Flexible Contact Probe."

In 1988 International Test Conference IEEE Proceedings, pages 608-614,in a paper entitled, "Very High Density Probing," C. Barsotti et al.describe current probe card testing technologies including the use of athin film hybrid diaphragm of controlled impedance signal, power andground conductors aligned to traces on a supporting printed circuitboard. This article describes present technology as limited to a fourmil pitch signal line spacing, which is indicative of the limitedpermissible variation in width of line traces.

A current state-of-the art membrane probe is disclosed in "MembraneProbe Card Technology" by B. Leslie and F. Matta in 1988 InternationalTest Conference, IEEE Proceedings, pages 601-607. The concept of themembrane probe is illustrated in FIG. I A flexible dielectric membranesupports a set of micro strip transmission lines that connect the testelectronics to the DUT. Each transmission line is formed by a conductortrace patterned on one side of the dielectric membrane. A thin metalfilm on the opposite side acts as a common ground plane. The width ofthe trace is chosen to obtain a desired line impedance to match aparticular device technology. Contact to the DUT, such as an individualintegrated circuit die in a wafer, is made by an array of micro contactbumps formed at the ends of the transmission lines via holes in themembrane.

The overall structure of a membrane probe is shown in FIG. 2. Themembrane of FIG. 1 is mounted on a printed circuit board carrier whichinterfaces with a test performance board via an appropriate connector. Aforced delivery spring mechanism, supported on the carrier, pushes onthe back surface of the membrane so as to protrude the contact bumpsbelow the plane of the carrier. In operation, the membrane probe card ismounted on any commonly used prober and the wafers are stepped onto theprobe in the same manner as with standard probes. Contact is made byraising the wafer toward the probe with a controlled overdrive. Thespring system is designed to produce a uniform contact force over theentire array.

Although the membrane probe has been generally successful, a number ofproblems have been encountered in its fabrication and use. For efficientfabrication, it is desirable to use a common format or layout for theprobe card for different purposes but, at the same time, it would bedesirable to be able to customize the membrane probe.

One way that a membrane probe can be customized is to terminate selectedtransmission lines by shorting to the ground plane. This can be done bylaser-drilling a hole through the dielectric membrane material andforming an electrically conductive VIA between the selected transmissionline and the ground plane through the hole. The problem in doing this,however, is that it is difficult, particularly using conventionalline-of-sight deposition techniques, to form a reliable conductorthrough such holes.

Various techniques are known for forming VIAs in printed circuit cards.U.S. Pat. No. 4,642,160 discloses a multi-layer printed circuit boardmanufacturing process. Metallic masks are formed on the surface of alayer of dielectric material, patterned to define VIA openings andirradiated from a laser light source to open VIAs in the dielectric intowhich an layer of copper is electrolessly deposited. EP Application No.0 227 903 A2 discloses a method of etching through a metal layer on ametal/polymer layered structure using the technique of ablativephotodecomposition (APD). APD relies on the use of ultraviolet laserradiation, which produces photochemical and other effects, as well asthermal effects, to remove irradiated material. The class of laser usedfor APD is commonly referred to as an excimer laser. IBM TechnicalDisclosure Bulletin, Vol. 29, No. 6, September 1986, pages 1862-64describes how profiles of VIA holes in a polymer substrate can be variedfrom nearly vertical to very much tapered when a metal mask and excimerlaser are used. P. E. Dyer et al. address the development and origin ofconical structures in XeCl laser ablative polyimide in Appl. Phys. Let.,49 (8), Aug. 25, 1986, pages 453-55. Detailed discussions of ablativecomposition of polyimide and other polymer films appear in V. Srinivasanet al., "Excimer Laser Etching of Polymers," J.Appl.Phys. 59 (11) June1986, pages 38, 61-67 and in J. H. Brannon et al., "Excimer LaserEtching of Polyimide," J.Appl.Phys. 58 (5) Sep. 1, 1985, pages 2036-43;B. J. Garrison et al., "Ablative Photodecomposition of Polymers,"J.Vac.Sci.Technol. A 3(3) May/June 1985, pp. 746-48; and J. R. Sheats,"Intensity-Dependent Photobleaching in Thin Polymer Films by ExcimerLaser: Lithographic Implications," App. Phys. Lett. 44 (10), May 15,1984, pages 1016-18. A paper by G. D. Poulin et al. entitled, "AVersatile Excimer Laser Processing System," SPIE Vol. 98, Excimer BeamApplications (1988), pages 17-23, describes the general state of the artto date of excimer laser processing of electrical circuitry on a polymersubstrate. None of these references appears to suggest solutions to theproblems discussed above.

Difficulties also arise in tailoring the transmission line impedance toeach test situation. B. Leslie et al. disclose that this can only bedone by varying the width of the transmission line trace. This measureis not adequate for all situations. Also, its use makes it harder tofabricate membrane probes with a common basic design. Differenttransmission line masks are required to vary the widths of the linetraces. Varying line width is also inconsistent with obtaining thenarrowest practical probe spacing, as is needed with increasingly denseintegrated circuitry. It would be preferable to have a way to alter thetransmission line impedance without varying line width or, to extend therange of possible impedance varying control, in combination with varyingline width.

Accordingly, a need remains for an improved membrane probe card andmethod of fabrication of membrane probes.

SUMMARY OF THE INVENTION

One object of the invention is to improve flexible circuits such asmembrane probes.

A second object is to improve the methods of fabrication of membraneprobes and similar flexible circuits.

Another object is to form controlled-impedance transmission lines inflexible circuits with a constant minimum pitch and variablecapacitance.

Yet another object is to enable selected transmission line traces on oneside of a membrane probe or other flexible circuit to be reliablyshorted to a ground plane or other circuit element on the opposite sideof the membrane dielectric material.

A further object is to control transmission line impedance other than byvarying the width of the line traces and to form through-VIAs in such away that line-of-sight deposition can be reliably used to form aconductor therethrough.

An additional object is to form structures other than holes in flexiblecircuit substrates.

One aspect of the invention is a novel method of forming a circuitconductor on an upper surface of a polymer substrate. A trench is formedin the substrate having a base spaced a predetermined distance below thesubstrate upper surface. The trench is contoured to include at least onelengthwise end wall defining a ramp extending at an obtuse angle betweenthe substrate upper surface and the base of the trench. An upperconductive layer is deposited on the upper surface and patterned to forma line trace including portions extending lengthwise continuously over aportion of the substrate upper surface, down the ramped end wall andalong the base of the trench. The trench is preferably formed andcontoured by successively removal of incremental thicknesses of polymermaterial of depthwise decreasing length from a predetermined length andwidth of the substrate. The incremental thicknesses of material removedare preferably dimensioned so that step-coverage of the conductive layerdeposited in the ramp is continuous.

The method is preferably carried out by laser-etching the surface of thesubstrate. This aspect of the invention includes selecting and operatingan excimer laser at a wavelength and energy density such that directinga beam from the laser at an upper surface of the substrate removes byablation a predetermined incremental thickness of polymer material toform a trench having a base spaced from the upper surface. The laser iscontrolled to contour a wall of the trench to a predetermined profile,such as a ramp.

A membrane probe can thus be formed on a flexible polymer membrane ofpredetermined thickness having planar parallel upper and lower surfaces,with a plurality of contact bumps protruding from a central area of thelower surface and a conductive ground plane covering a portion of thelower surface surrounding the central area. Conductive line traces onthe upper surface extending inward from a periphery of the membrane tosaid central area, including line traces electrically connected to thecontact bumps, can have a portion recessed into a trench in the uppersurface of the membrane. The recessed portion can contact a ground planeor other conductive layer on a lower side of the member, or can bespaced from the conductive layer by a predetermined dielectric thicknessdetermined by the depth of the trench to form a distributed capacitor.

The foregoing and other objects, features and advantages of theinvention will become more readily apparent from the following detaileddescription of a preferred embodiment which proceeds with reference tothe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a membrane probe card having amembrane probe mounted in position for testing an IC wafer or other DUT.

FIG. 2 is an enlarged sectional/perspective view of a portion of amembrane probe in accordance with the invention.

FIGS. 3 and 4 are cross-sectional views taken along lines 4--4 in FIG. 2illustrating successive steps in the method according to the inventionof fabrication of forming transmission line traces at different depthsin the membrane dielectric layer.

FIG. 5 is a longitudinal sectional view of the membrane probe of FIG. 4probe showing an increased capacitance line trace in accordance with theinvention.

FIG. 6 is a view similar to FIG. 5 showing a line trace shorted to theground plane through a ramped VIA in accordance with the invention.

FIG. 7 is a perspective view illustrating the method of the invention asused to form a VIA through the membrane dielectric layer to short a linetrace to a ground plane on the opposite side of the membrane.

FIG. 8 is an enlarged view of a portion of FIG. 7 detailing the profileof the ramped end wall.

DETAILED DESCRIPTION

Referring to FIG. 1, a circular membrane probe 10 is mounted for use ina probe card 12. The probe card includes an annular, multi-layer printedcircuit board-type carrier 14 having an upper surface 16 and a lowersurface 18. The outer periphery of the carrier is inletted with screwholes 20 for mounting the carrier on an annular interface ring 22.Terminations and by-passes and other circuit elements 24 may be formedon the upper surface of the carrier.

Upper and lower annular mounting rings 26, 28 are bolted to oppositesides of carrier 14 along the interior periphery thereof. Acircumferential margin 30 of the membrane probe 10 is secured in contactwith the lower surface 18 of carrier 14 by the lower ring 20. An annularforce delivery mechanism 32 is mounted centrally within the uppermounting ring 26. A central circular opening (not shown) in mechanism 32is transparent, as is a central rectangular opening in membrane probe10, to permit alignment of the probe to the device under test (DUT),such as a die on wafer 34. The wafer is supported on a platform (notshown) which is raised and lowered so that the upper surface of thewafer can contact the membrane probe. The membrane probe is contacted onits upper surface and stretched taut by the force delivery mechanism 32so as to protrude contact bumps 36 on the lower side thereof downwardinto contact with the wafer surface.

FIG. 1 shows in perspective a section of the membrane probe 10 embodyinga distributed capacitance line trace and a line trace-to-ground planecontact in accordance with the invention.

The membrane probe 10 is formed on a thin (e.g., 25 μm.) circular,polyimide (or other polymer) membrane 38 having a planar upper surface39 and a parallel lower surface. The lower surface of membrane 38 iscoated with a thin (9-18 μm.) plated or foil metal (e.g., copper) layer40. A rectangular window 42 is formed in the metal layer 40 in a centralregion of membrane probe 10 encompassing contact bumps 36. A set ofparallel transmission line traces 44, 46, 48 extend toward the center ofthe membrane probe from the periphery thereof, where contact is made tocircuitry on the multi-layer carrier 14.

Conventionally, all of these traces would be formed by patterning a thin(e g., 18 μm.) copper foil layer applied to the planar upper surface 39of membrane 38. The contact bumps are conventionally formed by nickelplating through circular VIAs to circular terminals 37 of selected linetraces. Optionally, the contact bumps are coated with a thin layer ofaluminum, platinum or gold.

In accordance with the invention, the upper surface 39 of membrane 38 isselectively contoured during fabrication of the membrane probe toterminate some traces and to modify the capacitance of other traces. Asshown in FIG. 2 and FIGS. 4 and 5, trace 44 is fabricated with anincreased capacitance while trace 48 is made with a normal or minimumcapacitance. Also, as shown in FIG. 2 and FIGS. 4 and 6, trace 46 isshown terminated to the ground plane 40.

Referring to FIGS. 3, 4 and 5, a lengthwise portion L of trace 44 isformed in a channel or trench 50 having a base 52 of width W and lengthL and opposite ramped ends 54, 56. Then, metal or other conductivematerial is deposited over the surface of the membrane 38, including inthe trench, and patterned to form the line traces. The preferredtechnique used to form and pattern the metal line traces is tosputter-deposit the metal over the membrane surface, mask the pattern ofthe traces, and etch away undesired areas of metal. Alternatively, anegative deposition technique can be used, in which the membrane surfaceis first masked, a seed layer is deposited over the areas in which thetraces are to be formed, including within the recesses, and then theremainder of thickness of the traces is built up by electroplating.

Both techniques use line-of-sight deposition, in which an adatom flux ofsource material is directed at the substrate surface on which thematerial to be deposited at an angle approximately normal to thesurface. Such techniques ordinarily cannot deposit material effectivelyon surfaces parallel or nearly parallel to the direction of deposition.Hence, the importance of structuring the trench so that deposited metal(or a seed layer) are applied reliably to the surface on which the linetraces are to be formed. This is accomplished by forming the trencheswith end walls, over which line trace continuity must be assured, thatare ramped at a slope sufficient to be coated by a line-of-sightdeposition technique. The slope of such end walls should be less thanabout 45° and preferably less than about 30°.

To increase the distributed capacitance along length L of trace 44, thetrench base 52 is recessed in the upper surface sufficiently to spaceline trace 44 a predetermined distance D₁ from ground plane 40. Trace 48is not recessed into the membrane 38 and so is spaced from the groundplane 40 by a distance D₀. The recess defines a dielectric thickness D₁which is less than the original dielectric thickness D₀ of the membrane38. For traces 44 and 48 of equal width, the ratio of capacitances ofportions of traces 44 and 48 of equal lengths is proportional to theratio D₀ divided by D₁, ignoring edge effects. Thus, if the base 52 oftrench 50 is recessed by one-half the thickness of the membrane, thenthe capacitance of a length of a trace 44 formed in the recess isapproximately twice that of a corresponding length and width of a trace48 formed atop the upper membrane surface 39. Besides varying thedielectric thickness D, capacitance can also be controlled by varyingthe length L of the trench.

Referring to FIGS. 6 and 7, a line trace such as trace 46 can beterminated by contact to the ground plane. A trench 60 is formed at thedesired location of termination with a base 62 at which the uppersurface of ground plane 40 is exposed, and opposite ramped ends 64, 66.End 64, over which continuity of the trace is important, should have aslope of less than about 60°, preferably of 45°. End 66 can have asteeper slope because line trace 46 terminates at that location. Thetrench 60 is relatively short, for example, 100 μm. at surface 39 andabout 20 μm. at its base 62. The line trace 46, including the portion intrench 60 contacting the ground plane at base 62, is formed in the samemanner and preferably in the same operations as line trace 44.

FIG. 7 also illustrates the manner in which the trenches 50, 60 areformed, as next explained. The trenches are formed using an excimerlaser. The beam of which is swept back and forth over an area of themembrane 38 to be recessed. Preferably, the laser has a slotted beamhaving a dimension 70 which defines the width W of the trench and adimension 72 aligned in the direction of the length L of the trench. Forexample, dimension 70 is 30 μm. and dimension 72 is 5 μm.

In general, the beam is swept back and forth in sweeps 74, 76, 78 ofprogressively decreasing length. The last, shortest sweep 78 defines thelength L of the bottom of the trench. The first, longest sweep 74 has alength (for example, 100 μm. in contact trench 60) that is determined bythe length L, the slopes of ends 64, 66 and the depth of the trench.

Referring to FIG. 8, the slope and smoothness of the ramped ends aredefined in incremental steps by the incremental thickness 80 of membranematerial removed during each sweep (preferably 1-2 μm.) and the changein length or offset 82 of each successive sweep. For a beam dimension 72of 5 μm. and an etch thickness 80 or ablation rate of 1.6 μm. per sweep,an offset 82 of 2 μm. has proven suitable. This offset can be dividedequally between both ends to form ramps of equal slope, as in the caseof ends 54, 56 (FIG. 5) or skewed to form ramps of unequal length as inthe case of ends 62, 64 (FIG. 6). An offset of 2 μm. provides an overlapof 3 μm. at the preferred laser sweep rate of 300 μm./sec and pulsedfrequency of 100 Hz. The ablation rate depends on the laser energy andpulse rate. The laser is selected to provide an energy sufficient toprovide bond dissociation for the material used in the membrane 38. Forpolyimide, these energies range from 3.6 eV (C-C) to 8 eV (C-N).

For lasers of a wavelength >351 nm. the photon energy is adsorbed asvibrational excitation, generating heat which "boils" the material out.This is thermal removal, which is difficult to localize. For excimerlasers having a wavelength <351 nm., energy is provided in excess ofthat needed to produce bond dissociation. The excess energy appears asheat or kinetic energy of the byproducts. Removal of material in thisinstance involves both thermal and ablation components. Another thermaleffect is reflow or cross-linking of the polymer material, whichinterferes with ablation. Hence, thermal effects should be minimizedduring etching in this process.

To avoid thermal effects, prior researchers in the use of excimer lasersslow the laser down. Most used are 249 nm. lasers operated at low pulsefrequencies: 3 Hz. 2 Hz. and less. This is not acceptable forproduction, which requires greater throughput, and the present inventionis best performed with different etch depths. Large pulse energies (2-10J./cm²) produce large steps/pulse. Smaller, smooth steps are needed forcontinuity of metallization. Another limitation is that the energy levelmust not melt the ground plane metal. For copper, this was found tobegin at above about 0.54 J./cm² and sets an upper limit of operation at0.72 J./cm.

The preferred laser for use in the present invention is one with awavelength of 308 nm. and which produces an energy level of about 4.0eV. The preferred operational window (in energy) was experimentallyfound to be between 0.18 J./cm² and 0.35 J./cm² at 100 Hz pulsefrequency, for polyimide. The optimum energy and pulse frequencyoperating ranges need to be determined for each material. The additionof oxygen improves the ablation process and widens the optimum window ofoperation. It is preferred to flood the locality of ablation with oxygen(O₂) while etching polymer material. Once the metal ground plane isreached, however, it is preferred to change to flooding the localitywith nitrogen (N₂) to keep from oxidizing the metal surface. Theforegoing operational parameters and sequence of operation to formcapacitive line trace trenches 50, trenches for through-VIAS 60, andother recessed patterns as may be useful in polymer substrates arepreferably computer controlled.

A second technique can be used to obtain smooth steps as an alternativeto ablating progressively shorter trenches. This technique includesablating coincident shallow squares (or circles) and then smoothing thesteps by defocusing the laser. The latter step lowers the energy densityand causes reflowing of polymer material without further oblation. Thistechnique could allow faster removal rates (deeper steps) or customizingthe profile.

Having illustrated and described the principles of our invention in apreferred embodiment and examples thereof, it should be readily apparentto those skilled in the art that the invention can be modified inarrangement, detail and application without departing from suchprinciples. We claim all modifications coming within the spirit andscope of the accompanying claims.

We claim:
 1. A membrane probe comprising:a flexible polymer membranehaving planer parallel upper and lower surfaces; a plurality of contactbumps protruding from a central area of the lower surface; a conductiveground plane covering a portion of the lower surface surrounding thecentral area; and a plurality of conductive line traces on the uppersurface extending inward from a periphery of the membrane to saidcentral area, including a subset of line traces each electricallyconnected to one of the contact bumps; the membrane having a thicknesssufficient to insulate the conductive line traces from conductive groundplane; at least one of the line traces having a portion recessed intothe upper surface of the membrane to position said line trace portionrelative to the ground plane on the opposite side of the membrane at adepthwise spacing less than the thickness of the membrane.
 2. A membraneprobe according to claim 1 in which each recessed portion of line traceis formed in a trench extending lengthwise thereof, the trench having abase spaced from the upper surface of the membrane defining thedepthwise spacing of the line trace portion relative to the groundplane.
 3. A probe according to claim 2 in which the trench has a depthnot less than the thickness of the membrane so that the line traceportion therein contacts the ground plane.
 4. A probe according to claim2 in which the trench has a depth less than the thickness of themembrane so that the line trace portion therein capacitively couples tothe ground plane.
 5. A probe according to claim 4 in which the linetrace portion in the trench has a distributed capacitance defined by thelength and width of the recessed portion of the line trace and aremaining thickness of the polymer material underlying the trench base.6. A probe according to claim 2 in which the trench includes a rampintegrally formed in the polymer membrane to form a substantially smoothtransition of the line trace from the upper surface into the trench.